The invention of this application is concerned with a method of producing, defect free monocrystalline layers of semiconductor materials on insulating layers. In particular the invention is concerned with the formation of thin monocrystalline layers of silicon on insulators, the so called SOI structure.
In the production of thin layers of monocrystalline silicon on insulating layers a thin layer of polycrystalline or amorphous silicon is deposited on an insulating substrate particularly monocrystalline silicon, oxidized silicon or quartz. By localized heating of the silicon layer a small molten zone is produced in the silicon layer which molten zone is scanned across the silicon layer while the underlying substrate is heated. As the molten layer of silicon recrystallizes the silicon layer is converted to a monocrystalline layer.
Such a procedure is well-known in the art, examples in being shown in European Application EPA-0129261, Anthony, U.S. Pat. No. 4,585,493, Klein, U.S. Pat. No. 4,590,130, J. Sakurai, J. Electrochem Soc. Solid-State Science and Technology, pp. 1485-1488 (July, 1986), C.L. Bleil et al, Mat. Res. Soc. Symp. Proc. Vol. 35, pp. 389-692 and Japanese Kokai 61-19116.
The problem with these prior arts is that while the zone melting methods disclosed produce monocrystalline layers the monocrystalline layers contain an undesirable concentration of defects. These defects are found primarily to be the presence of sub-grain or low angle grain boundaries. These sub-grain boundaries are believed to arise due to stresses in the growing crystal. These stresses are believed to be chemical impurities, surface, surface roughness, volume expansion or freezing of thermal gradients.
The Japanese Kokai, C. L. Bleil et al. and J. Sakurai show examples of the use of low melting glasses such as phosphosilicate glass, borophosphosilicate glass or corning 7059 as substrates underlying the recrystallizable silicon layer. It is taught in these references that by the use of these substrates an alternate mechanism for stress relief is provided so that the formation of low angle grain boundaries will be eliminated or significantly reduced. However, while the use of these substrates results in the reduction of some formation of low angle grain boundaries the formation of these low angle grain boundaries has not been eliminated or even significantly reduced.
The method significantly reducing the formation of low angle grain boundaries is disclosed in copending U.S. Application No. 084,657 filed Aug. 11, 1987 and commonly assigned. According to the method disclosed in this copending application, the formation of low angle grain boundaries may be significantly reduced so that thin defect free monocrystalline layers of semiconductor materials of significantly large areas and insulators may be produced. According to the invention, as described in this application, the method involves depositing a thin monocrystalline or amorphous layer of a semiconductor material on a low softening point temperature insulating substrate having a softening point of at least 10.degree. C. below the melting point of a semiconductor material and the slope of its viscosity versus temperature curve being less than zero and greater than negative infinity, heating the layer of the semiconductor material by means of a zone heating source in such a manner as to provide a convex solid-liquid interface on the layer of the semiconductor material while the layered structure is scanned relative to the zone heating source, the substrate is heated to its annealing point and the scanning speed of the structure relative to the zone heating source is controlled in order to cause the convex solid liquid interface to move along the layer of semiconductor material while the surface of the substrate imposing the layer of the semiconductor material is liquified over the solid-liquid interface.
While this method clearly increases the distance between the defects resulting in the recrystallization to the presence of sub-grain boundaries this method does not prevent the formation of the sub-grain boundaries. By use of this method a 500 .mu.m wide layer of defect free monocrystalline zone was produced with a length of 2 mm. However, for many purposes it is highly desirable that area of defect free monocrystalline silicon be significantly increased.